Yersinia pestis is a human pathogen with multifactorial virulence factors, which inhibit the host response to infection, allowing lethal growth within the host. Virulent strains of Y. pestis possess a virulence plasmid, which encodes the genes for a type-III apparatus and type-Ill cytotoxins. Type-III cytotoxins are injected directly into the host cell by the bacterium to elicit pathology, YopE, YopH, YopM, YpkA, YopJ, and YopT. YopT contributes to the inhibition of phagocytosis of professional phagocytes by modulating their actin cytoskeleton. While the molecular basis for the reorganization of the actin cytoskeleton is not clear, YopT appears to interact with Rho, a monomeric cellular GTPase. This goal of this proposal is to characterize structure-function properties of YopT. It is anticipated that YopT will possess features that are common to type-III cytotoxins, but will also possess features that are unique. These studies will provide new insights to neutralize Y. pestis during human infections and allow the PI to begin studies on the type-IIIcytotoxins produced by Y. pestis, using expertise in the study of both conventional bacterial toxins and type-III cytotoxins. [unreadable] [unreadable] While no longer a general health threat, Y. pestis is believed an agent that can be used in biological warfare or bioterrorist attacks, a class A pathogen. The ability of YopT to interfere with the phagocytic process of cells of the innate immune system is a key step in the infectious process of Y. pestis. Elucidation of the molecular and cellular action of YopT will provide insight to prevent early colonization of humans by this pathogen. [unreadable] [unreadable] [unreadable]